I believe you have a legitimate fear that if you tell her she has to choose between the marriage and the fire academy, she'll go with the academy. Believe it or not, I empathize with you, Coconut.
As I've said, there is no compromising with an A. Don't make the mistake some men make about emotional affairs. Men seem to be more concerned about physical affairs than the emotional one. For women, an emotional A is just as serious, b/c so much of a woman's emotions are tied to her soul. I don't say this to make you feel even worse, but hopefully prevent you from not taking it as seriously.......and classifying it as just an EA.
With that said, I don't know how any plan would work very well, as long as they are attending the same classes and going to the same firehouse. Every time she see him across the room, it feeds her addiction. That is why there has to be no visual contact, vocal contact, written, social media, nothing. A drug addict, or any type of addiction, will not stop as long as s/he is feeding the addiction.
She could call or text you every ten minutes when she is in class or the firehouse. Wouldn't work b/c OM is there. NC of any kind is the only way. Which would basically require one of them to transfer elsewhere.....or drop out completely. I don't know if they could be on different schedules, classes, etc.
The choice is hers to make. Your part is to set a boundary and stand behind what you say. Something like I suggested earlier. Telling what you cannot do, like living in a MR with three people. If she is serious about wanting a new R, then she must make some difficult choices (which shouldn't be difficult when choosing you M or something else).
I think she will offer to keep you posted, etc., while she's out, but it doesn't work if OM is there.
If she refuses to leave the fire academy, the ball is back in your court. Will you tell her you will prepare for S/D, or will you live in an open M?
And I think I read a poster to say it might be too early to go NC?! My goodness, why would you continue to wait until it had turned into a PA? The longer they they engage in any contact, the less likely the A will end.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!